It is a hard one, and I can see it from both sides.
Our DCs will tell how it is to be the younger GCs, when the older 2 get all the attention. Slightly different for them as we all lived within 10 miles of GPs.
We are currently in the process of planning to move almost 200 miles to be near our youngest GCs. Our older ones have had us around since birth, giving childcare when mum had to go back to work . Now that they are much older, and both parents work from home much of the time, we are free to move to be near the younger ones.
It is, however, our decision, and we are hopefully able to afford a house to move into (albeit smaller than our current one, which we no longer need) and a static caravan on a site near the Older GC, so that we can visit regularly